1. Field of Use
The invention concerns method and apparatus for the production of a combustible product derived from waste material.
2. Description of the Prior Art
Waste products, and in particular solid domestic and trade refuse has for many years been disposed of by one or other of several methods namely by tipping, burning or, more recently, composting. All of these disposal methods have inherent disadvantages.
For example, if waste is to be tipped it is first necessary to locate an area which can be used and in most instances such an area is removed from the source of the waste by a considerable distance and thus transportation costs are high. Furthermore the area used for tipping is unsightly and may, unless precautions are taken, become a breeding ground for example, for rodents. Eventually a tip becomes full and a new site must be located, but before the area used as a tip becomes re-usable for some other purpose a considerable period of time must elapse. In some cases the period will extend to several years.
Waste disposal by burning requires initial capital expenditure in the construction of incinerators and the burning of waste in many instances requires the use of supplementary fuel. The heat produced is usually completely wasted since no means of efficiently using it is available and in any event the burning of waste in incinerators is a low efficiency method of producing heat which requires specially designed and thus expensive equipment. Incineration produces a large amount of ash which must subsequently be disposed of and this in itself increases the cost of waste disposal by incineration.
The method of waste disposal by composting can be used for the disposal of solids and non-toxic liquids and the compost when produced must still be tipped or stored prior to disposal for agricultural purposes. Clearly the compost produced cannot be used as a humus on grazing land without careful screening (requiring additional plant) since it will contain materials such as broken glass and solid materials which are harmful to livestock.
The need exists therefore to provide a waste disposal means which is ecologically acceptable. Taking all of the abovementioned factors into account and also bearing in mind the fact that the world demand for fuel is increasing while the supply of fuel is decreasing it is clearly desirable to utilize all available sources of fuel in order to effect a reduction in the demand for conventional fuels such as coal, oil and gas and the applicant has appreciated the feasibility of producing a commercially acceptable fuel derived from waste materials.
It is known to the applicant that proposals have been made in the past for the production of a fuel derived from waste material and that attempts have been made to put such proposals into effect.
For example, it has been proposed to treat refuse by a process such as ball or hammer milling to break down solid waste into a condition such that, by the addition of binders, the broken down waste can be pelletised or briquetted. Such processes, which clearly require the carrying out of a multiplicity of process steps, are expensive and in many cases the end product is not capable of being burnt efficiently. For example, it is known to use non-combustible materials as binders for the broken down waste when pelletising or briquetting but such binders have the disadvantage that they reduce the calorific value of the end product and create relatively large amounts of ash which must then be disposed of. In addition of course to the steps of breaking down the waste and adding binders, it is essential to provide apparatus to compress the material to produce pellets or briquettes and, in some cases to remove excess liquids.
It is also found that the storage of briquetted or pelletised waste derived fuels may present problems since if these fuels are externally stored the affect of rain, frost, snow or simply lengthy periods of storage will result in deterioration of the fuel thus it may be found necessary to provide under cover storage facilities, again adding cost to the fuel.
One proposal for the production of a waste material derived fuel is disclosed in U.S. Pat. No. 1,486,488 in which it is proposed to mix together garbage, sawdust, rubbish, weeds and other similar material of a fibrous or cellular combustible nature and to mix these materials in ground-up condition with for example swamp clay or peat-muck. To this mass it is said to be preferable to add a quantity of oil, the proportion of the latter being determined to some extent of the quantity of garbage in the ground-up mixture. The mass produced is then compressed to produce a mass from which the major proportion of the moisture has been expelled and the mass is then encased in, for example, paper tubes. This method of producing fuel requires a multiplicity of process steps all of which must be taken into account when calculating production costs.
It is known that many solid fuel furnaces, such as travelling grate furnaces will only burn carefully graded fuel efficiently and consequently the equipment provided for handling the fuel and feeding it to the furnace is designed specifically to handle the graded fuel. If it is desired to use briquetted waste derived fuel the latter must be produced in such sizes as to meet the fuel size requirement of the type of furnace in which it is to be burnt and thus specially designed briquetting equipment is necessary and costly. It should be borne in mind in this context that simply to desintegrate refuse with a view to feeding it to furnaces of the kind under discussion will not obviate the problem outlined above for the reason that even if desintegrated waste could be fed to the furnace the size variation of the material, together with density variations between batches of material and differences in calorific value would result in uneven combustion. In addition, light weight materials, such as pieces of paper, would blow off the furnace bed and burn above it. Similar problems would exist in fluidised bed furnaces.
Briefly stated therefore the amount of waste derived fuel that is now produced is very small in relation to total fuel demands due to the following factors:
(a) The high cost of conversion of waste PA1 (b) The high cost of handling and storage PA1 (c) The inefficient combustion